Mac OS X PPC Shellcode Tricks
H D Moore
hdm[at]metasploit.com
Last modified: 05/09/2005
0) Foreword
Abstract:
Developing shellcode for Mac OS X is not particularly difficult, but there are
a number of tips and techniques that can make the process easier and more eff
ective. The independent data and instruction caches of the PowerPC processor
can cause a variety of problems with exploit and shellcode development. The
common practice of patching opcodes at run-time is much more involved when the
instruction cache is in incoherent mode. NULL-free shellcode can be improved by
taking advantage of index registers and the reserved bits found in many
opcodes, saving space otherwise taken by standard NULL evasion techniques. The
Mac OS X operating system introduces a few challenges to unsuspecting
developers; system calls change their return address based on whether they
succeed and oddities in the Darwin kernel can prevent standard execve()
shellcode from working properly with a threaded process. The virtual memory
layout on Mac OS X can be abused to overcome instruction cache obstacles and
develop even smaller shellcode.
Thanks:
The author would like to thank B-r00t, Dino Dai Zovi, LSD, Palante, Optyx, and
the entire Uninformed Journal staff.
1) Introduction
With the introduction of Mac OS X, Apple has been viewed with mixed feelings by
the security community. On one hand, the BSD core offers the familiar Unix
security model that security veterans already understand. On the other, the
amount of proprietary extensions, network-enabled software, and growing mass of
advisories is giving some a cause for concern. Exploiting buffer overflows,
format strings, and other memory-corruption vulnerabilities on Mac OS X is a
bit different from what most exploit developers are familiar with. The
incoherent instruction cache, combined with the RISC fixed-length instruction
set, raises the bar for exploit and payload developers.
On September 12th of 2003, B-r00t published a paper titled "Smashing the Mac
for Fun and Profit". B-root's paper covered the basics of Mac OS X shellcode
development and built on the PowerPC work by LSD, Palante, and Ghandi. This
paper is an attempt to extend, rather than replace, the material already
available on writing shellcode for the Mac OS X operating system. The first
section covers the fundamentals of the PowerPC architecture and what you need
to know to start writing shellcode. The second section focuses on avoiding NULL
bytes and other characters through careful use of the PowerPC instruction set.
The third section investigates some of the unique behavior of the Mac OS X
platform and introduces some useful techniques.
2) PowerPC Basics
The PowerPC (PPC) architecture uses a reduced instruction set consisting of
32-bit fixed-width opcodes. Each opcode is exactly four bytes long and can only
be executed by the processor if the opcode is word-aligned in memory.
2.1) Registers
PowerPC processors have thirty-two 32-bit general-purpose registers (r0-r31)
PowerPC 64-bit processors have 64-bit general-purpose registers, but still use
32-bit opcodes, thirty-two 64-bit floating-point registers (f0-f31), a link
register (lr), a count register (ctr), and a handful of other registers for
tracking things like branch conditions, integer overflows, and various machine
state flags. Some PowerPC processors also contain a vector-processing unit
(AltiVec, etc), which can add another thirty-two 128-bit registers to the set.
On the Darwin/Mac OS X platform, r0 is used to store the system call number, r1
is used as a stack pointer, and r3 to r7 are used to pass arguments to a system
call. General-purpose registers between r3 and r12 are considered volatile and
should be preserved before the execution of any system call or library
function.
;;
;; Demonstrate execution of the reboot system call
;;
main:
li r0, 55 ; #define SYS_reboot 55
sc
2.2) Branches
Unlike the IA32 platform, PowerPC does not have a call or jmp instruction.
Execution flow is controlled by one of the many branch instructions. A branch
can redirect execution to a relative address, absolute address, or the value
stored in either the link or count registers. Conditional branches are
performed based on one of four bit fields in the condition register. The count
register can also be used as a condition for branching and some instructions
will automatically decrement the count register. A branch instruction can
automatically set the link register to be the address following the branch,
which is a very simple way to get the absolute address of any relative location
in memory.
;;
;; Demonstrate GetPC() through a branch and link instruction
;;
main:
xor. r5, r5, r5 ; xor r5 with r5, storing the value in r5
; the condition register is updated by the . modifier
ppcGetPC:
bnel ppcGetPC ; branch if condition is not-equal, which will be false
; the address of ppcGetPC+4 is now in the link register
mflr r5 ; move the link register to r5, which points back here
2.3) Memory
Memory access on PowerPC is performed through the load and store instructions.
Immediate values can be loaded to a register or stored to a location in memory,
but the immediate value is limited to 16 bits. When using a load instruction on
a non-immediate value, a base register is used, followed by an offset from that
register to the desired location. Store instructions work in a similar fashion;
the value to be stored is placed into a register, and the store instruction
then writes that value to the destination register plus an offset value.
Multi-word memory instructions exist, but are considered bad practice to use,
since they may not be supported in future PowerPC processors.
Since each PowerPC instruction is 32 bits wide, it is not possible to load a
32-bit address into a register with a single instruction. The standard method
of loading a full 32-bit value requires a load-immediate-shift (lis) followed
by an or-immediate (ori). The first instruction loads the high 16 bits, while
the second loads the lower 16 bits Some people prefer to use
add-immediate-shift against the r0 general purpose register. The r0 register
has a special property in that anytime it is used for addition or substraction,
it is treated as a zero, regardless of the current value 64-bit PowerPC
processors require five separate instructions to load a 32-bit immediate value
into a general-purpose register. This 16-bit limitation also applies to
relative branches and every other instruction that uses an immediate value.
;;
;; Load a 32-bit immediate value and store it to the stack
;;
main:
lis r5, 0x1122 ; load the high bits of the value
; r5 contains 0x11220000
ori r5, r5, 0x3344 ; load the low bits of the value
; r5 now contains 0x11223344
stw r5, 20(r1) ; store this value to SP+20
lwz r3, 20(r1) ; load this value back to r3
2.4) L1 Cache
The PowerPC processor uses one or more on-chip memory caches to accelerate
access to frequently referenced data and instructions. This cache memory is
separated into a distinct data and instruction cache. Although the data cache
operates in coherent mode on Mac OS X, shellcode developers need to be aware of
how the data cache and the instruction cache interoperate when executing
self-modifying code.
As a superscalar architecture, the PowerPC processor contains multiple
execution units, each of which has a pipeline. The pipeline can be described as
a conveyor belt in a factory; as an instruction moves down the belt, specific
steps are performed. To increase the efficiency of the pipeline, multiple
instructions can put on the belt at the same time, one behind another. The
processor will attempt to predict which direction a branch instruction will
take and then feed the pipeline with instructions from the predicted path. If
the prediction was wrong, the contents of the pipeline are trashed and correct
instructions are loaded into the pipeline instead.
This pipelined execution means that more than one instruction can be processed
at the same time in each execution unit. If one instruction requires the output
of another, a gap can occur in the pipeline while these dependencies are
satisfied. In the case of store instruction, the contents of the data cache
will be updated before the results are flushed back to main memory. If a load
instruction is executed directly after the store, it will obtain the
newly-updated value. This occurs because the load instruction will read the
value from the data cache, where it has already been updated.
The instruction cache is a different beast altogether. On the PowerPC platform,
the instruction cache is incoherent. If an executable region of memory is
modified and that region is already loaded into the instruction cache, the
modifed instructions will not be executed unless the cache is specifically
flushed. The instruction cache is filled from main memory, not the data cache.
If you attempt to modify executable code through a store instruction, flush the
cache, and then attempt to execute that code, there is still a chance that the
original, unmodified code will be executed instead. This can occur because the
data cache was not flushed back to main memory before the instruction cache was
filled.
The solution is a bit tricky, you must use the "dcbf" instruction to invalidate
each block of memory from the data cache, wait for the invalidation to complete
with the "sync" instruction, and then flush the instruction cache for that
block with "icbi". Finally, the "isync" instruction needs to be executed before
the modified code is actually used. Placing these instructions in any other
order may result in stale data being left in the instruction cache. Due to
these restrictions, self-modifying shellcode on the PowerPC platform is rare
and often unreliable.
The example below is a working PowerPC shellcode decoder included with the
Metasploit Framework (OSXPPCLongXOR).
;;
;; Demonstrate a cache-safe payload decoder
;; Based on Dino Dai Zovi's PPC decoder (20030821)
;;
main:
xor. r5, r5, r5 ; Ensure that the cr0 flag is always 'equal'
bnel main ; Branch if cr0 is not-equal and link to LMain
mflr r31 ; Move the address of LMain into r31
addi r31, r31, 68+1974 ; 68 = distance from branch -> payload
; 1974 is null eliding constant
subi r5, r5, 1974 ; We need this for the dcbf and icbi
lis r6, 0x9999 ; XOR key = hi16(0x99999999)
ori r6, r6, 0x9999 ; XOR key = lo16(0x99999999)
addi r4, r5, 1974 + 4 ; Move the number of words to code into r4
mtctr r4 ; Set the count register to the word count
xorlp:
lwz r4, -1974(r31) ; Load the encoded word into memory
xor r4, r4, r6 ; XOR this word against our key in r6
stw r4, -1974(r31) ; Store the modified work back to memory
dcbf r5, r31 ; Flush the modified word to main memory
.long 0x7cff04ac ; Wait for the data block flush (sync)
icbi r5, r31 ; Invalidate prefetched block from i-cache
subi r30, r5, -1978 ; Move to next word without using a NULL
add. r31, r31, r30
bdnz- xorlp ; Branch if --count == 0
.long 0x4cff012c ; Wait for i-cache to synchronize (isync)
; Insert XORed payload here
.long (0x7fe00008 ^ 0x99999999)
3) Avoiding NULLs
One of the most common problems encountered with shellcode development in
general and RISC processors in particular is avoiding NULL bytes in the
assembled code. On the IA32 platform, NULL bytes are fairly easy to dodge,
mostly due to the variable-length instruction set and multiple opcodes
available for a given task. Fixed-width opcode architectures, like PowerPC,
have fixed field sizes and often pad those fields with all zero bits.
Instructions that have a set of undefined bits often set these bits to zero as
well. The result is that many of the available opcodes are impossible to use
with NULL-free shellcode without modification.
On many platforms, self-modifying code can be used to work around NULL byte
restrictions. This technique is not useful for single-instruction patching on
PowerPC, since the instruction pre-fetch and instruction cache can result in
the non-modified instruction being executed instead.
3.1) Undefined Bits
To write interesting shellcode for Mac OS X, you need to use system calls. One
of the first problems encountered with the PowerPC platform is that the system
call instruction assembles to 0x44000002, which contains two NULL bytes. If we
take a look at the IBM PowerPC reference for the 'sc' instruction, we see that
the bit layout is as follows:
010001 00000 00000 0000 0000000 000 1 0
------ ----- ----- ---- ------- --- - -
A B C D E F G H
These 32 bits are broken down into eight specific fields. The first field (A),
which is 5 bits wide, must be set to the value 17. The bits that make up B, C,
and D are all marked as undefined. Field E is must either be set to 1 or 0.
Fields F and H are undefined, and G must always be set to 1. We can modify the
undefined bits to anything we like, in order to make the corresponding byte
values NULL-free. The first step is to reorder these bits along byte boundaries
and mark what we are able to change.
? = undefined
# = zero or one
[010001??] [????????] [????0000] [00#???1?]
The first byte of this instruction can be either 68, 69, 70, or 71 (DEFG). The
second byte can be any character at all. The third byte can either be 0, 16,
32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, or 240 (which
contains '0', 'P', and 'p', among others). The fourth value can be any of the
following values: 2, 3, 6, 7, 10, 11, 14, 15, 18, 19, 22, 23, 26, 27, 30, 31,
34, 35, 38, 39, 42, 43, 46, 47, 50, 51, 54, 55, 58, 59, 62, 63. As you can see,
it is possible to create thousands of different opcodes that are all treated by
the processor as a system call. The same technique can be applied to almost any
other instruction that has undefined bits. Although the current line of PowerPC
chips used with Mac OS X seem to ignore the undefined bits, future processors
may actually use these bits. It is entirely possible that undefined bit abuse
can prevent your code from working on newer processors
;;
;; Patching the undefined bits in the 'sc' opcode
;;
main:
li r0, 1 ; sys_exit
li r3, 0 ; exit status
.long 0x45585037 ; sc patched as "EXP7"
3.2) Index Registers
On the PowerPC platform, immediate values are encoded using all 16 bits. If the
assembled value of your immediate contains a NULL, you will need to find another
way to load it into the target register. The most common technique is to first
load a NULL-free value into a register, then substract that value minus the
difference to your immediate.
;;
;; Demonstrate index register usage
;;
main:
li r7, 1999 ; place a NULL-free value into the index
subi r5, r7, 1999-1 ; substract our value minus the target
; the r5 register is now set to 1
If you have a rough idea of the immediate values you will need in your
shellcode, you can take this a step further. Set your initial index register to
a value, that when decremented by the immediate value, actually results in a
character of your choice. If you have two distant ranges (1-10 and 50-60), then
consider using two index registers. The example below demonstrates an index
register that works for the system call number as well as the arguments,
leaving the assembled bytes NULL-free. As you can see, besides the four bytes
required to set the index register, this method does not significantly increase
the size of the code.
;;
;; Create a TCP socket without NULL bytes
;;
main:
li r7, 0x3330 ; 0x38e03330 = NULL-free index value
subi r0, r7, 0x3330-97 ; 0x3807cd31 = system call for sys_socket
subi r3, r7, 0x3330-2 ; 0x3867ccd2 = socket domain
subi r4, r7, 0x3330-1 ; 0x3887ccd1 = socket type
subi r5, r7, 0x3330-6 ; 0x38a7ccd6 = socket protocol
.long 0x45585037 ; patched 'sc' instruction
3.3) Branching
Branching to a forward address without using NULL bytes can be tricky on
PowerPC systems. If you try branching forward, but less than 256 bytes, your
opcode will contain a NULL. If you obtain your current address and want to
branch to an offset from it, you will need to place the target address into the
count register (ctr) or the link register (lr). If you decide to use the link
register, you will notice that every valid form of "blr" has a NULL byte. You
can avoid the NULL byte by setting the branch hint bits (19-20) to "11"
(unpredictable branch, do not optimize). The resulting opcode becomes
0x4e804820 instead of 0x4e800020 for the standard "blr" instruction.
The branch prediction bit (bit 10) can also come in handy, it is useful if you
need to change the second byte of the branch instruction to a different
character. The prediction bit tells the processor how likely it is that the
instruction will result in a branch. To specify the branch prediction bit in
the assembly source, just place '-' or '+' after the branch instruction.
4) Mac OS X Tricks
This section describes a handful of tips and tricks for writing shellcode on
the Mac OS X platform.
4.1) Diagnostic Tools
Mac OS X includes a solid collection of development and diagnostic tools, many
of which are invaluable for shellcode and exploit development. The list below
describes some of the most commonly used tools and how they relate to shellcode
development.
Xcode: This package includes 'gdb', 'gcc', and 'as'. Sadly, objdump is not
included and most disassembly needs to be done with 'gdb' or 'otool'.
ktrace: The ktrace and kdump tools are equivalent to strace on Linux and truss
on Solaris. There is no better tool for quickly diagnosing shellcode
bugs.
vmmap: If you were looking for the equivalent of /proc/pid/maps, you found it.
Use vmmap to figure out where the heap, library, and stacks are mapped.
crashreporterd: This daemon runs by default and creates very nice crash dumps
when a system service dies. Invaluable for finding 0-day in Mac OS X
services. The crashdump logs can be found in /Library/Logs/CrashReporter.
heap: Quickly list all heaps in a process. This can be handy when the
instruction cache prevents a direct return and you need to find an
alternate shellcode location.
otool: List all libraries linked to a given binary, disassemble mach-o
binaries, and display the contents of any section of an executable or
library. This is the equivalent of 'ldd' and 'objdump' rolled into a
single utility
4.2) System Call Failure
An interesting feature of Mac OS X is that a successful system call will return
to the address 4 bytes after the end of 'sc' instruction and a failed system
call will return directly after the 'sc' instruction. This allows you to
execute a specific instruction only when the system call fails. The most common
application of this feature is to branch to an error handler, although it can
also be used to set a flag or a return value. When writing shellcode, this
feature is usually more annoying than anything else, since it boosts the size
of your code by four bytes per system call. In some cases though, this feature
can be used to shave an instruction or two off the final payload.
4.3) Threads and Execve
Mac OS X has an undocumented behavior concerning the execve() system call
inside a threaded process. If a process tries to call execve() and has more
than one active thread, the kernel returns the error EOPNOTSUPP. After a closer
look at kernexec.c in the Darwin XNU source code, it becomes apparent that for
shellcode to function properly inside a threaded process, it will need to call
either fork() or vfork() before calling execve().
;;
;; Fork and execute a command shell
;;
main:
_fork:
li r0, 2
sc
b _exitproc
_execsh: ; based on ghandi's execve
xor. r5, r5, r5
bnel _execsh
mflr r3
addi r3, r3, 32 ; 32
stw r3, -8(r1) ; argv[0] = path
stw r5, -4(r1) ; argv[1] = NULL
subi r4, r1, 8 ; r4 = {path, 0}
li r0, 59
sc ; execve(path, argv, NULL)
b _exitproc
_path:
.ascii "/bin/csh" ; csh handles seteuid() for us
.long 0
_exitproc:
li r0, 1
li r3, 0
sc
4.4) Shared Libraries
The Mac OS X user community tends to have one thing in common -- they keep
their systems up to date. The Apple Software Update service, once enabled, is
very insistent about installing new software releases as they become available.
The result is that nearly every single Mac OS X system has the exact same
binaries. System libraries are often loaded at the exact same virtual address
across all applications. In this sense, Mac OS X is starting to resemble the
Windows platform.
If all processes on all Mac OS X system have the same virtual addresses for the
same libraries, Windows-style shellcode starts to become possible. Assuming you
can find the right argument-setting code in a shared library, return-to-library
payloads also become much more feasible. These libraries can be used as return
addresses, similar to how Windows exploits often return back to a loaded DLL.
Some useful addresses are listed below:
0x90000000: The base address of the system library (libSystem.B.dylib), most
of the function locations are static across all versions of OS X.
0xffff8000: The base address of the "common" page. A number of useful
functions and instructions can be found here. These functions
include memcpy, sysdcacheflush, sysicacheinvalidate, and bcopy.
The following NULL-free example uses the sysicacheinvalidate function to flush
1040 bytes from the instruction cache, starting at the address of the payload:
;;
;; Flush the instruction cache in 32 bytes
;;
main:
_main:
xor. r5, r5, r5
bnel main
mflr r3
;; flush 1040 bytes starting after the branch
li r4, 1024+16
;; 0xffff8520 is __sys_icache_invalidate()
addis r8, r5, hi16(0xffff8520)
ori r8, r8, lo16(0xffff8520)
mtctr r8
bctrl
5) Conclusion
In the first section, we covered the fundamentals of the PowerPC platform and
described the syscall calling convention used on the Darwin/Mac OS X platform.
The second section introduced a few techniques for removing NULL bytes from
some common instructions. In the third section, we presented some of the tools
and techniques that can be useful for shellcode development.
Bibliography
B-r00t PowerPC / OSX (Darwin) Shellcode Assembly.
http://packetstormsecurity.org/shellcode/PPC_OSX_Shellcode_Assembly.pdf
Bunda, Potter, Shadowen Powerpc Microprocessor Developer\'s Guide.
http://www.amazon.com/exec/obidos/tg/detail/-/0672305437/
Steve Heath Newnes Power PC Programming Pocket Book.
http://www.amazon.com/exec/obidos/tg/detail/-/0750621117/
IBM PowerPC Assembler Language Reference.
http://publib16.boulder.ibm.com/pseries/en_US/aixassem/alangref/mastertoc.htm